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United States Patent |
6,114,846
|
Bosselmann
,   et al.
|
September 5, 2000
|
Optical measuring method and device for measuring a magnetic alternating
field with an expanded measuring range and good linearity
Abstract
Linearly polarized measuring light is split after traversing a Faraday
sensor device into two partial light signals having planes of polarization
inclined at 45.degree.. A measured signal, which is proportional to the
tangent of the Faraday rotation angle, is derived from the two partial
light signals.
Inventors:
|
Bosselmann; Thomas (Erlangen, DE);
Menke; Peter (Botzow, DE)
|
Assignee:
|
Siemens Aktiengesellschaft (Munich, DE)
|
Appl. No.:
|
117006 |
Filed:
|
January 8, 1999 |
PCT Filed:
|
January 3, 1997
|
PCT NO:
|
PCT/EP97/00022
|
371 Date:
|
January 8, 1999
|
102(e) Date:
|
January 8, 1999
|
PCT PUB.NO.:
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WO97/26547 |
PCT PUB. Date:
|
July 24, 1997 |
Foreign Application Priority Data
| Jan 18, 1996[DE] | 196 01 727 |
Current U.S. Class: |
324/96; 324/244.1 |
Intern'l Class: |
G01R 033/032; G01R 015/24; G01R 019/02 |
Field of Search: |
324/96,244.1
356/364
250/227.17
|
References Cited
U.S. Patent Documents
4755665 | Jul., 1988 | Ulmer, Jr. et al.
| |
4894608 | Jan., 1990 | Ulmer, Jr. | 324/96.
|
4973899 | Nov., 1990 | Jones et al. | 324/96.
|
5656934 | Aug., 1997 | Bosselmann | 324/244.
|
5764046 | Jun., 1998 | Bosselmann | 324/96.
|
5844409 | Dec., 1998 | Bosselmann et al. | 324/96.
|
5847560 | Dec., 1998 | Bosselmann et al. | 324/96.
|
Foreign Patent Documents |
0 088 419 | Sep., 1983 | EP.
| |
0 208 593 | Jan., 1987 | EP.
| |
89/00701 | Jan., 1989 | WO.
| |
91/01500 | Feb., 1991 | WO.
| |
95/10046 | Apr., 1995 | WO.
| |
Other References
Hirsch et al. "Fiber Optic Current Sensor With Optical Analog Transmission"
Sensor 93 Conference Report IV vol. 11.1, pp. 137-144 .sup.* Jan. 1993.
|
Primary Examiner: Strecker; Gerard
Attorney, Agent or Firm: Kenyon & Kenyon
Claims
What is claimed is:
1. A method for measuring an alternating magnetic field, comprising the
steps of:
a) coupling a linearly polarized measuring light into a sensor device, the
sensor device being arranged in the alternating magnetic field and
exhibiting the Faraday effect;
b) causing the linearly polarized measuring light to traverse the sensor
device at least once;
c) splitting the linearly polarized measuring light into a first linearly
polarized partial light signal and a second linearly polarized partial
light signal, wherein a direction of polarization of the first linearly
polarized partial light signal and a direction of polarization of the
second linearly polarized partial light signal are at an angle relative to
one another, the angle being substantially an odd multiple of 45.degree.;
d) converting the first linearly polarized partial light signal into a
first electrical intensity signal, the first electrical intensity signal
being a measure of a light intensity of the first linearly polarized
partial light signal;
e) converting the second linearly polarized partial light signal into a
second electrical intensity signal, the second electrical intensity signal
being a measure of a light intensity of the second linearly polarized
partial light signal;
f) determining a first alternating signal component and a first constant
signal component from the first electrical intensity signal, the first
alternating signal component including substantially all frequency
components of the alternating magnetic field;
g) determining a second constant signal component from the second
electrical intensity signal, wherein the first constant signal component
and the second constant signal component includes substantially none of
the frequency components of the alternating magnetic field;
h) deriving a first intensity-normalized signal from a quotient of the
first alternating signal component and the first constant signal
component;
i) deriving a second intensity-normalized signal from a quotient of the
second electrical intensity signal and the second constant signal
component; and
j) deriving a measuring signal for the alternating magnetic field, the
measuring signal being proportional to a quotient of the first
intensity-normalized signal and the second intensity-normalized signal.
2. The method according to claim 1, further comprising the step of:
forming from the measuring signal a measure of a root-mean-square value of
the alternating magnetic field.
3. The method according to claim 1, wherein the alternating magnetic field
is generated by an alternating electrical field, and wherein the method
further comprises the step of measuring the alternating electrical field.
4. A system for measuring an alternating magnetic field, comprising:
a) a sensor device capable of exhibiting the Faraday effect;
b) an arrangement for coupling a linearly polarized measuring light into
the sensor device;
c) an arrangement for splitting the linearly polarized measuring light,
after the linearly polarized measuring light has traversed the sensor
device at least once, into a first linearly polarized partial light signal
and a second linearly polarized partial light signal, a direction of
polarization of the first linearly polarized partial light signal and a
direction of polarization of the second linearly polarized partial light
signal are at an angle relative to one another, the angle being
substantially an odd multiple of 45.degree.;
d) an arrangement for converting the first linearly polarized partial light
signal into a first electrical intensity signal, the first electrical
intensity signal being a measure of a light intensity of the first
linearly polarized partial light signal, the arrangement for converting
further for converting the second linearly polarized partial light signal
into a second electrical intensity signal, the second electrical intensity
signal being a measure of a light intensity of the second linearly
polarized partial light signal;
e) an arrangement for determining a first alternating signal component and
a first constant signal component from the first electrical intensity
signal, the first alternating signal component essentially including all
frequency components of the alternating magnetic field, the arrangement
for determining further for determining a second constant signal component
from the second electrical intensity signal, wherein none of the first
constant signal component and the second constant signal component
includes the frequency components of the alternating magnetic field; and
f) an arrangement for deriving a first intensity-normalized signal from a
quotient of the first alternating signal component and the first constant
signal component, for deriving a second intensity-normalized signal from a
quotient of the second electrical intensity signal and the second constant
signal component, and for deriving a measuring signal for the alternating
magnetic field, the measuring signal being proportional to a quotient of
the first intensity-normalized signal and the second intensity-normalized
signal.
5. The system according to claim 4, further comprising an arrangement for
forming from the measuring signal a measure of a root-mean-square value of
the alternating magnetic field.
6. The system according to claim 4, wherein the alternating magnetic field
is generated by an alternating electrical field, and wherein the system
further comprises an arrangement for measuring the alternating electrical
field.
Description
FIELD OF THE INVENTION
The present invention relates to a method and an arrangement for measuring
an alternating magnetic field. An alternating magnetic field is understood
to be a magnetic field which has in its frequency spectrum only frequency
components differing from zero.
BACKGROUND OF THE INVENTION
Optical measuring arrangements for measuring an electrical current in an
electrical conductor are known which are based on the magneto-optic
Faraday effect, and are therefore also designated as magneto-optic current
transformers. In a magneto-optic current transformer, linearly polarized
measuring light is transmitted through a Faraday sensor device which is
arranged in the vicinity of the electrical conductor and includes an
optically transparent material exhibiting the Faraday effect. Because of
the Faraday effect, the magnetic field generated by the current causes a
rotation of the plane of polarization of the measuring light by a
rotational angle .rho., which is proportional to the path integral over
the magnetic field along the path covered by the measuring light in the
sensor device. The constant of proportionality is the Verdet constant V.
The Verdet constant V is generally a function of the material and the
temperature of the sensor device, as well as of the wavelength of the
measuring light employed. In general, the sensor device surrounds the
electrical conductor, so that the measuring light runs at least once
around the electrical conductor in a virtually closed path. The rotational
angle .rho. is, in this case, essentially directly proportional to the
amplitude I of the current to be measured, in accordance with the relation
.rho.=N.multidot.V.multidot.I (1),
N being the number of revolutions of the measuring light around the
electrical conductor. The Faraday rotational angle .rho. is determined
polarimetrically by performing a polarization analysis of the measuring
light running through the sensor device, in order to obtain a measuring
signal for the electrical current.
It is known for the purpose of polarization analysis to use an analyzer to
decompose the measuring light, after it has traversed the sensor device,
into two linearly polarized light components L1 and L2 having planes of
polarization, which are directed perpendicularly with respect to one
another. A polarizing beam splitter can be used as the analyzer for this
polarization analysis. Specifically, some of the types of polarizing beam
splitters that can be used in this analysis include a Wollaston prism or a
simple beam splitter having two downstream polarizers whose axes of
polarization are rotated by .pi./2 or 90.degree. with respect to one
another. Each of the two light components L1 and L2 is converted by one
assigned photoelectric transducer into, in each case, an electrical
intensity signal T1 or T2, which is proportional to the light intensity of
the light component L1 or L2, respectively. A measuring signal
T=(T1-T2)/(T1+T2) (3)
which corresponds to the quotient of a difference and the sum of the two
intensity signals T1 and T2, as described in PCT Application No. WO
95/10046, is formed from these two electrical signals.
Disregarding interference effects, this measuring signal T is given by
T=sin(2.pi.+.zeta.)=sin(2.multidot.N.multidot.V.multidot.I+.zeta.)(4),
.zeta. being an offset angle for I=0 A, which is a function of the angle
between the plane of polarization of the measuring light on being coupled
into the Faraday element and a distinctive intrinsic optical axis of the
analyzer.
Although, according to equation (1), the Faraday measuring angle .rho. is
itself a linear, and thus unique, function of the current I, according to
equation (4) the measuring signal T is a unique function of the measuring
angle .rho. only over an angular range of at most .pi./2 (or 90.degree.).
Consequently, it is possible using these polarimetric magneto-optic
current transformers to measure uniquely only those electrical currents
which lie in a current measuring range (current measuring interval) MR
with an interval length of
.vertline.MR.vertline.=.pi./(2.multidot.N.multidot.V) (5)
It is clear from equation (5) that the magnitude .vertline.MR.vertline. of
the current measuring range MR of a magneto-optic current transformer can
be set by the selection of materials having different Verdet constants V
for the Faraday element and/or by the number N of revolutions of the
measuring light around the electrical conductor. A larger current
measuring range is obtained by setting the product N.multidot.V in the
denominator smaller. However, such a selection of a larger current
measuring range MR is inescapably attended by a reduced measuring
resolution MA of the current transformer for a given display resolution.
The measuring resolution MA is defined in this case as the absolute value
.vertline.MS.vertline. of the measuring sensitivity MS of the current
transformer. The measuring sensitivity MS corresponds to the gradient of
the characteristic curve of the magneto-optic current transformer at an
operating point, and in the case of two-channel evaluation, is given
according to equation (4) by
MS=dT/dI=2.multidot.N.multidot.V.multidot.cos(2.multidot.N.multidot.V.multi
dot.I+.zeta.) (6).
It is immediately evident from equation (6) that reducing the product
N.multidot.V leads, in the case of both evaluation methods, to a reduction
in the measuring resolution MA=.vertline.MS.vertline..
European Patent Application No. 088 419 describes a magneto-optic current
transformer in which two Faraday glass rings, which are made of Faraday
materials having different Verdet constants and thus each having
inherently different current measuring ranges, are arranged parallel to
one another about a common electrical conductor. Each Faraday glass ring
is assigned a transmission unit for transmitting linearly polarized
measuring light into the glass ring and a two-channel evaluation unit for
calculating a respective measuring signal for each Faraday rotational
angle. The two measuring signals of the two evaluation units are fed to an
OR gate, which determines a maximum signal from the two measuring signals.
This maximum signal is used to switch between the measuring ranges of the
two glass rings. Different measuring ranges of the two glass rings can
also be obtained given the same glass material for the two glass rings by
employing measuring light of different wavelengths. In this context, the
wavelength dependence of the Faraday rotation is utilized.
The publication entitled "Fiber Optic Current Sensor With Optical Analog
Transmission", SENSOR 93 Conference Report IV Vol. 11.1, pages 137 to 144,
describes a magneto-optical current transformer for protective purposes
for measuring alternating currents, in which, after traversing a Faraday
optical fiber, linearly polarized light is split into two partial light
signals and each of these light signals is fed to an analyzer. The
intrinsic axes (axes of polarisation) of the two analyzers are directed at
an angle of 45.degree. or 58.degree. is relative to one another. The light
intensities passed by the analyzers are not normalized until division by
their constant components, which are obtained by peak value rectification.
Subsequently, a product of the normalized signals is formed and this
product is then differentiated. The Faraday rotational angle is obtained
directly by integration. As a result, a signal is obtained which is
proportional to the current and, therefore, is not subject to measuring
range limitations. However, this method is comparatively costly.
European Patent No. 208 593 describes a magneto-optic current transformer
in which, after traversing a Faraday optical fiber surrounding an
electrical conductor, linearly polarized measuring light is split by a
beam splitter into two partial light signals and each of these partial
light signals is fed to an analyzer. The intrinsic axes of the two
analyzers are directed at an angle of 0.degree. and 45.degree.,
respectively, relative to the coupling polarization of the measuring
light. This produces a first, sinusoidal signal at the output of one
analyzer, and a second, cosinusoidal signal at the output of the other
analyzer. These two signals are, in each case, non-unique, oscillating
functions of the current in the electrical conductor, which are
phase-shifted with respect to one another by an angle of 90.degree.. A
unique measuring signal is now composed from these two non-unique signals
by comparing the sign and the absolute values of the measuring values of
the first, sinusoidal signal and of the second, cosinusoidal signal. As
soon as the absolute values of the sine and cosine are equal, that is to
say given an integral multiple of 45.degree., a switch is made, as a
function of the sign of sine and cosine, from a unique branch of the
first, sinusoidal signal to a unique branch of the second, cosinusoidal
signal, or vice versa. The measuring range of this known magneto-optic
current transformer is thus, in principle, unlimited. However, the method
is an incremental method, with the result that the operating point for
current zero must be reset anew whenever there is a failure of the
electronics of the current transformer.
SUMMARY OF THE INVENTION
An object of the present invention is to specify a method and an
arrangement, having an extended measuring range and good linearity, for
measuring an alternating magnetic field.
In order to achieve this object, linearly polarized measuring light is
coupled into a sensor device which exhibits the Faraday effect and is
arranged in the alternating magnetic field, at least during the measuring
operation. The measuring light traverses the sensor device at least once
and is thereafter fed into two linearly polarized partial light signals
whose directions of polarization are directed, relative to one another, at
an angle of essentially an odd multiple of 45.degree. or .pi./4. The two
partial light signals are, in each case, converted into an electrical
intensity signal which is a measure of the light intensity of the
associated partial light signal. An alternating signal component and a
constant signal component are determined from a first of the two
electrical intensity signals, and a constant signal component is
determined from a second of the two intensity signals. The alternating
signal component contains essentially all the frequency components of the
alternating magnetic field. The two constant signal components, in
contrast, contain essentially no frequency components of the alternating
magnetic field. A measuring signal is now derived for the alternating
magnetic field and is proportional to a quotient of two
intensity-normalized signals, a first of the two intensity-normalized
signals corresponding to the quotient of the alternating signal component
and the constant signal component of the first intensity signal, and a
second of the two intensity-normalized signals corresponding to the
quotient of the second intensity signal and the constant signal component
thereof. This measuring signal is, on the one hand, virtually independent
of undesirable intensity fluctuations of the measuring light and is, on
the other hand, a unique function over an angular range of approximately
.pi. for the Faraday rotational angle .rho., by which the plane of
polarization of the measuring light in the sensor device is rotated based
on the magnetic field, for example over the open angular range of
T/2<.rho.<+.pi.2 . Furthermore, the measuring signal has an excellent
linearity in a large range, around an operating point situated in the
middle of the measuring range.
According to the present invention, as a measure of the root-mean-square
value of the alternating magnetic field, a root-mean-square value is
formed from the measuring signal, for the purpose of precision
measurement.
In order to measure an alternating electrical field, the sensor device is
arranged in the alternating magnetic field generated inductively by the
alternating current.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of an arrangement for measuring an alternating
magnetic field and, in particular, for measuring an alternating electrical
current in accordance with an exemplary embodiment of the invention.
FIG. 2 illustrates the root-mean-square value of the measured current as a
function of the Faraday measuring angle.
DETAILED DESCRIPTION
Represented in FIG. 1 is an exemplary embodiment of an arrangement for
measuring an alternating magnetic field H which can be generated, in
particular, by an electrical current I in an electrical conductor 2. A
sensor device 3 exhibiting the magneto-optic Faraday effect is arranged in
alternating magnetic field H. In the embodiment represented, sensor device
3 is formed by means of a single-mode optical fiber which preferably
surrounds electrical conductor 2 in the form of a measuring winding of at
least one turn. It is preferred to provide an annealed optical fiber which
is distinguished by low linear birefringence and virtually negligible
circular birefringence. However, sensor device 3 can also be formed from
one or more solid bodies, preferably made from glass, exhibiting the
Faraday effect, and can, in particular, surround electrical conductor 2 as
a polygonal annular body. Means are provided for coupling linearly
polarized measuring light L into sensor device 3. The direction of
polarization of the electrical field strength vector of measuring light L
during coupling into sensor device 3 is denoted below as the direction of
coupling polarization of measuring light L. The means for coupling
measuring light L into sensor device 3 can, as represented, contain a
light source 9 and a polarizer 10 for linearly polarizing the light of
light source 9, or also a light source which is itself linearly polarized,
such as a laser diode, for example. In the embodiment represented, the
axis of polarization (transmission axis) of polarizer 10 prescribes the
direction of coupling polarization of measuring light L. The linearly
polarized measuring light L coupled into sensor device 3 traverses sensor
device 3 and is fed, after traversing sensor device 3, to a beam splitter
4. Beam splitter 4 decomposes measuring light L into two light components
L1' and L2' having the same polarization. For example, beam splitter 4 can
be formed with a semi-transparent mirror inclined at an angle of
preferably 45.degree. to the direction of propagation of measuring light
L. A first polarizer 5, which forms a first partial light signal L1
projected onto its axis P1 of polarization, is arranged in the optical
path (beam path) of first light component L1'. A second polarizer 6, which
forms a second partial light signal L2 projected onto its associated axis
P2 of polarization, is arranged in the optical path of second light
component L2'. Axis P1 of polarization of first polarizer 5 and axis P2 of
polarization of second polarizer 6 form an angle of at least approximately
.alpha.=(2n+1).multidot.45.degree. or
.alpha.=(2n+1).multidot.(.pi./4) (7)
relative to one another, where n is a whole number. Axis P1 of polarization
of first polarizer 5 is preferably directed at an angle of at least
approximately +45.degree. or +.pi./4, or -45.degree. or -.pi./4 relative
to the direction of coupling polarization of measuring light L, and axis
P2 of polarization of second polarizer 6 is directed at an angle of
0.degree. or 0 relative to the direction of coupling polarization of
measuring light L.
The two component signals L1 and L2 are fed to an assigned photoelectric
transducer 7 or 8, respectively. Each photoelectric transducer 7 and 8
converts associated light signals L1 and L2, respectively, into an
electrical intensity signal S1 or S2, respectively, which is a measure of
the intensity of the respective partial light signal L1 or L2. Generally,
electrical intensity signal S1 or S2 is proportional to the light
intensity of associated partial light signal L1 or L2, respectively. The
output of first photoelectric transducer 7 is then electrically connected
to the input on a high-pass filter 11 and to the input of a low-pass
filter 12. High-pass filter 11 forms an alternating signal component A1 of
first intensity signal S1,and low-pass filter 12 forms a constant signal
component D1 of this first intensity signal S1. The separating frequency
of high-pass filter 11 and low-pass filter 12 are set such that
alternating signal component A1 contains all the frequency components of
alternating magnetic field H, and constant signal component D1 is
independent of alternating magnetic field H. Alternating signal component
A1 of first intensity signal S1 is fed from an output of high-pass filter
11 to a first input of a divider 14. Constant signal component D1 of first
intensity signal S1 is fed from an output of low-pass filter 12 to a
second input of divider 14.
Divider 14 now forms quotient signal A1/D1 of alternating signal component
A1 to constant signal component D1 of first intensity signal S1. This
quotient signal A1/D1 is an intensity-normalized signal, that is to say it
is independent of changes in the intensity of measuring light L, for
example, owing to intensity fluctuations of light source 9 or attenuation
losses in the light path of measuring light L or of first partial light
signal L1. The output of second photoelectric transducer 8 is electrically
connected to the input of a low-pass filter 13 and to a first input of a
divider 15. Lowpass filter 13 forms a constant signal component D2 of
second intensity signal S2. The separating frequency of lowpass filter 13
is set such that constant signal component D2 contains no frequency
components of alternating magnetic field H. There is now present at an
output of divider 15 a quotient signal S2/D2 which corresponds to the
quotient of second intensity signal S2 and constant signal component D2
thereof. This quotient signal S2/D2 is also an intensity-normalized
signal, and is thus independent of changes in intensity in measuring light
L and in second partial light signal L2. Since changes in intensity in the
light paths of two partial light signals L1 and L2 are compensated by the
intensity normalization, multimode fibers can also be used to transmit two
partial light signals L1 and L2. Two normalized signals A1/D1 and S2/D2
are now fed, in each case, to an input of a further divider 16. Divider 16
forms the quotient of two normalized signals A1/D1 and S2/D2 as measuring
signal
M=(A1/D1)/(S2/D2) (8)
which can be tapped at an output 30 of the arrangement.
This measuring signal M is similar to function tan(.rho.) of Faraday
rotation angle .rho. by which the direction of polarization (plane of
polarization) of measuring light (L) is rotated in sensor device 3 based
on alternating magnetic field H. However, tangent function tan(.rho.) is a
unique function of rotational angle .rho. over an angular interval having
an interval length of approximately .pi., specifically for
-.pi./2+2m.pi.<.rho.<+.pi./2 +2m.pi., with m being a whole number. The
result is a measuring range which is virtually twice as large as in the
case of the measuring signals obtained in accordance with the related art,
which are proportional to sin(2.rho.).
In a preferred embodiment, means 17 are provided for forming
root-mean-square value M.sub.eff of measuring signal M, which functions as
a measure of the amplitude (absolute value) of alternating magnetic field
H, or as a measure of root-mean-square value I.sub.eff of an electrical
current I in electrical conductor 2. FIG. 2 shows root-mean-square value
M.sub.eff of measuring signal M for a sinusoidal electrical current
I=2.sup.0.5 I.sub.eff sin (.omega.t), plotted over an angular range of
0.degree. to approximately 60.degree. of root-mean-square value
.rho..sub.eff of Faraday rotational angle .rho.-2.sup.0.5 .rho..sub.eff
sin (.omega.t). Root-mean-square value I.sub.eff of electrical current I
is then obtained from equation .rho..sub.eff =2 NV I.sub.eff with N the
number of turns of the fiber coil (measuring winding) and Verdet constant
V of sensor device 3. Any analog or digital circuit known per se can be
used to form root-mean-square value M.sub.eff.
Root-mean-square value M.sub.eff of measuring signal M can also be
subjected to a subsequent linearization, preferably with the aid of a
digital signal processor 18. Linearized root-mean-square value
M.sub.eff.sup.lin, then linearly dependent on rotational angle .rho., is
applied to an output 20. Of course, root-means-quare value M.sub.eff
itself can also be applied to an output (not represented).
Instead of high-pass filter 11, it is also possible, for the purpose of
forming alternating signal component A1 of first intensity signal S1, to
provide a subtractor which forms difference S1-D1 between first intensity
signal S1 and its constant signal component D1, which is formed by lowpass
filter 12. This difference corresponds precisely to constant signal
component A1. Conversely, instead of low-pass filter 12, it is also
possible, for the purpose of forming constant signal component D1 of first
intensity signal Si, to provide a subtractor which forms difference S1-A1
between first intensity signal S1 and its alternating signal component A1,
which is formed by high-pass filter 11. This difference corresponds
precisely to constant signal component D1. Furthermore, low-pass filter 13
can also be replaced by a high-pass filter for the purpose of forming an
alternating signal component A2 of second intensity signal S2, and by a
subtractor for the purpose of forming constant signal component D2 of
second intensity signal S2 by subtracting alternating signal component A2
from second intensity signal S2. Finally, the analog filters represented
can also be replaced by digital filters and analog-to-digital convertors
connected upstream.
Of course, instead of analog dividers 14, 15 and 16, it is also possible to
provide digital calculating means as arithmetic means for deriving
measuring signal M, in accordance with relation (8). As explained above,
the value of relation (8) is based on alternating signal component A1 and
constant signal component D1 of first intensity signal S1 and from second
intensity signal S2 and a constant signal component D2 thereof. Such a
digital calculating means may include a microprocessor or a digital signal
processor having an analog-to-digital converter connected upstream. It is
preferable to provide both digital filters and digital arithmetic means.
The analog-to-digital conversion is then performed upstream of the digital
filters.
The optical coupling of the various optical components of the measuring
arrangement is preferably supported by collimator lenses (Grin lenses),
not depicted, for focusing the light.
Instead of the type of transmission shown in FIG. 1, in which measuring
light L traverses sensor device 3 only once, it is also possible to
provide an arrangement of the reflection type in which, after traversing
sensor device 3 a first time, measuring light L is retroflected into
sensor device 3 with the aid of a mirror and traverses sensor device 3 a
second time in the opposite direction, before it is fed to beam splitter
4.
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